Caterpillar Halitosis

A tobacco-eating caterpillar creates the equivalent of smoker’s breath to scare off predators.

Transcript

The bad-breath defense. I’m Bob Hirshon and this is Science Update.

A large caterpillar called the tobacco hornworm uses something like smoker’s breath to scare hungry spiders. This according to Ian Thomas Baldwin, founding director of the Max Planck Institute for Chemical Ecology in Germany.

The tobacco-eating caterpillars consume massive doses of nicotine, which makes them toxic to local wolf spiders. Baldwin’s team discovered that the caterpillars exhale some of that nicotine through tiny holes in their bodies.

Baldwin:And so when the spider attacks this caterpillar, it’s getting a faceful of bad breath that’s full of nicotine. That is a sign, for the spider, that it’s going to probably get poisoned if it eats this caterpillar.

Baldwin’s team showed that when the caterpillars were rendered incapable of exhaling nicotine, or when they ate bioengineered low-nicotine tobacco, the spiders gobbled them right up. I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

Nictoine, the addictive chemical in tobacco products, is also a poison. It stimulates nerve receptors that communicate with muscles. In moderate doses, this stimulation produces the “buzz” smokers seek out, but at higher doses, it can overwhelm and shut down those nerves—with fatal results. This is true not just for people, but for most other organisms as well.

The tobacco hornworm is an exception. Since tobacco is its primary food source, it's developed a way to protect itself from nicotine. Unlike most creatures, it's able to keep the nicotine sequestered in its gut without having it poison the rest of its system. So it eventually just poops out most of the nicotine it takes in. And it takes in a lot: enough to kill six or seven humans, every single day.

A major predator in some of the hornworms' habitat is the wolf spider—an active, agile hunter that would seemingly love an easy target like a fat caterpillar. However, the researchers had noticed that wolf spiders avoid hornworms that feed on tobacco leaves. This makes sense, insofar as the caterpillar's high nicotine content would kill the spider. What was surprising, however, was that while most toxic insects advertise themselves with bright, showy colors—which predators quickly learn to avoid—this caterpillar is green and blends in with the tobacco plant.

These experiments show that it's the caterpillar's breath, not its coloring, that warns the spiders to stay away. The breath escapes through tiny holes called spiracles, found on the caterpillar's body, which release waste gases. In the caterpillar's case, the waste gas includes a substantial puff of nicotine.

The researchers conducted two experiments that support this hypothesis. First, when caterpillars fed on genetically engineered tobacco plants that were low in nicotine, wolf spiders suddenly developed a taste for them. The same happened when the researchers shut down the biological mechanism that allows the caterpillars to exhale nicotine. In other words, when there was little to no nicotine in the caterpillar breath, either because their bodies couldn't release it or their diets didn't contain it, the spiders no longer found them repellent. In fact, they found them quite tasty.

Now try and answer these questions:

What's unusual about the tobacco hornworm?

How did the researchers determine that the caterpillar's “breath” repels predatory spiders?

Suppose the spiders ate the caterpillars when they were fed low-nicotine tobacco, but unlike in this experiment, they still avoided them when they could no longer exhale nicotine. What conclusions would you draw from that?